Pin to Savon 2.2.x

Savon 2.3.x has a breaking change in that it changes:

<tns:Objects xsi:type="tns:DataExtensionObject">
  ...
</tns:Objects>

into

<tns:Objects>
...
</tns:Objects>
<tns:attributes>
  <tns:Objects>
    <xsi:type>tns:DataExtensionObject</xsi:type>
  </tns:Objects>
</tns:attributes>
Bump to 0.0.10